1. The environments that are available to us are harsher. Seasteading? The Antarctic? The Moon or Mars? Come on, do you really want to live in any of those places? Sure, Virginia may have had a bit more malaria than England but it has a) solid ground, b) it never drops below -50 C and c) it has a 21% O2 atmosphere. Consequently, it takes a more complex and developed economy to allow survival in the harsher land. And even Jamestown wasn't self-sustaining until the third ship full of people and supplies arrived. (More on Jamestown and Mars here.) And think: how big would a colony in Antarctica have to be in order to be self-sufficient, and make all the equipment they need to survive, not to mention to trade with the rest of the world? (See below for places that are much more amenable to humans than Antarctica and frontiers ripe for settlement right now, but somehow are still not filling up with colonists, in some cases despite the local government's attempt to draw them.)

2. There is a bigger skills gap between the median person and what a colonist needs to know. Even if a new island appeared above the ocean and had a nice temperate climate, few of us (especially in the developed world) would be able to take advantage of it. In Jamestown, people plowed and planted fields, hunted, chopped wood, and built small structures. That's pretty much what they were doing at home except for the building part. Even in the developing world, the gap between the skillsets required of someone living day-to-day versus what they would need to do in a terra nova is much wider than what the settlers of the New World faced, or the Polynesians that expanded across the Pacific. And the hunter-gatherers who crossed the land bridge from Siberia to North America almost certainly didn't even know they were on a new continent (and why would they have cared?)

3. We're just more comfortable. Yes, there are still people in desperate poverty, but not as many of us three centuries ago. The median human is much happier, and if they move, they have more information about which countries offer better opportunities, rather than helping to build a country from scratch.

The exceptions I alluded to above are Siberia and the Canadian interior and the Australian Outback. That's a significant chunk of the Earth's land surface. Seriously, if you think that there is no more wilderness and no more frontiers, just buy a few coats and a hunting rifle and move to the Yukon. West Australia is the size of America's Western and Pacific time zones combined and has a population of 2 million, 1.5 million of which are in one city, and at least near the coast a Mediterranean climate quite like California's, and massive mineral wealth to boot - and even with all that, the Australian government has been desperately and unsuccessfully trying to get people to settle it. In any of these places you can quite easily meet Daniel Boone's requirement of refusing to live anywhere that you can see the smoke from your neighbor's chimney. But you won't do this, despite any belly-aching you might have done along these lines. Why not? Because you have a good life already and you have no idea how to hunt, that's why.